Steel piston ring

ABSTRACT

In a corner portion ( 2 ) defined between an outer peripheral surface ( 4 ) and a lower-side surface ( 3 ) of a steel piston ring ( 1 ), a groove ( 5, 6 ) having either an approximately quadrangular shape with a length of one side of 0.05 to 0.4 mm or an approximately quarter circular shape having a radius of 0.05 to 0.4 mm is formed. A sharp edge ( 9 ) formed by the groove ( 5, 6 ) performs an oil scraping function and the groove ( 5, 6 ) works to reduce an oil pressure of a second land or a third land clearance between the cylinder bore and the piston.

TECHNICAL FIELD

[0001] The present invention relates to piston ring, and moreparticularly to an improved piston ring made of a steel material.

BACKGROUND ART

[0002] A piston ring used in a reciprocating internal combustion engineis, based on a function thereof, classified into a pressure ring and anoil ring. In an automobile engine or the like, the pressure rings aregenerally composed of a first pressure ring which is positioned closestto a combustion chamber side and a second pressure ring which ispositioned at a side below the first pressure ring (crank chamber side).

[0003] The first pressure ring is positioned close to the combustionchamber and hence, the first pressure ring is subjected to hightemperature and not sufficiently lubricated. Accordingly, the firstpressure ring is required to have high strength and good slidingcharacteristics. Conventionally, a piston ring made of cast iron whichhas a hard chromium plating layer formed on a sliding surface thereofhas been popularly used. Recently, however, along with the increase ofdemand for higher speed, higher output and lower fuel consumption withrespect to an internal combustion engine, requirements for piston ringare getting severer year by year. Under such circumstances, with respectto the first pressure ring, the conventional cast-iron piston ringhaving the chromium plating layer is not sufficient in view of thestrength and the wear resistance. Accordingly, in place of the cast-ironpiston ring, a steel piston ring made of martensitic stainless steelwith a nitriding has been widely used.

[0004] On the other hand, the sliding circumstance of the secondpressure ring is not so severe compared with the first pressure ring andhence, a cast-iron piston ring made of such as flaky graphite cast ironhas been used up to now and a shift to steel material has not been takenplace. However, from a viewpoint of the prevention of the increase ofthe global greenhouse effect, an automobile engine is required tosatisfy the further reduction of the fuel consumption so that thereduction of weight of the piston ring is desired.

[0005] In view of such circumstances, there has been observed a trendthat a base material of the second pressure ring is shifted from castiron to steel in the same manner as the first pressure ring thusproducing a steel piston ring which is thinner and lighter than thecast-iron piston ring.

[0006] Here, while the first pressure ring and the second pressure ringare required to have a function of sealing a combustion gas, thesepressure rings are also required to have a function of scraping alubricant applied to an inner peripheral surface of a cylinder by an oilring toward a crankcase side as an important function thereof.Accordingly, these pressure rings are required to satisfy a conditionthat the pressure rings easily ride on the applied lubricant during apiston upward stroke (when the piston moves upwardly toward a cylinderhead) and easily scrape the lubricant during the piston downward stroke(when the piston moves downwards).

[0007] To satisfy such a requisite, it is desired that an outerperipheral sliding surface of the piston ring is formed into abarrel-face surface or has a diameter thereof decreased and inclinedtoward a combustion chamber side thus forming a so-called taper-facesurface, and a lower-surface-side corner portion which is defined by alower side surface and an outer peripheral sliding surface forms aso-called sharp edge having an acute angle. Since the conventionalcast-iron material exhibits the good machinability, it is easy tomachine the piston ring in such a shape.

[0008] However, the machinability of the steel material is inferior tothat of the cast iron material. Accordingly, it has been found that, inproducing the above-mentioned piston ring having a sharp edge, when thebase material of the first pressure ring and the second pressure ring isshifted from the cast iron to the steel, more burrs are frequentlygenerated in grinding and lathe turning of the piston ring and hence,the productivity is remarkably decreased.

[0009] In general, a steel wire material which constitutes a rawmaterial of a steel piston ring having a rectangular cross section ismanufactured by drawing and/or rolling a steel material as described ina publication entitled “Manufacturing method of piston ring material”and accorded “Japanese Patent Laid-open Publication No. 035173/1989”.The wire material is coiled and cut in a ring shape. Grinding, latheturning and the like are done thereafter so as to manufacture pistonrings having a given size.

[0010] Accordingly, as shown in FIG. 1 and FIG. 3, at a stage that onlythe coiling is done, each corner portion 2 of the piston ring 1maintains a round corner with a radius of R which the wire materialoriginally has. Accordingly, to manufacture the piston ring having asharp edge, it is necessary to machined out the whole side surface orthe whole outer peripheral surface of the piston ring piece by an amountof R by grinding or lathe turning.

[0011] However, as mentioned previously, since the steel material istough, it is difficult to remove swarf or turnings from the piston ringthus increasing the generation of so-called “burrs”. Accordingly,compared with the use of the cast-iron material, to suppress thegeneration of the burrs, it is necessary to provide means such asreducing a grinding speed, making a depth of cut small and the like thusremarkably decreasing the machining productivity.

[0012] On the other hand, to enhance an oil scraping function of apiston ring, as disclosed in Japanese Patent Laid-open Publication No.124204/2001, a piston ring, of which a groove is formed at a cornerportion defined between an outer peripheral sliding surface and a lowerside surface, but interrupted at abutting end portions, has been known.This groove is extending axially up to a barrel-face crest portion.However, a depth of the groove in the radial direction is equal to ormore than 0.5 mm and hence, this constitutes one of usual piston ringshaving an interrupted undercut shape.

[0013] Although the publication does not specifically describe materialof the piston ring, in view of a machining operation to manufacture thepiston ring of the interrupted undercut shape, the piston ring may bemade of cast iron. This can be supported by a fact that there is nospecific suggestion with respect to a steel piston ring in thepublication. When this known technique is applied to a steel pistonring, as mentioned previously, problems still remain with respect to thesuppression of the generation of burrs and the machining productivity.

[0014] Further, to mention performances of the piston ring, it isnecessary to provide additional shapes to the piston ring and hence,following problems arise.

[0015] (1) Since the undercut is large, it is necessary to interrupt theundercut at abutting end portions to cope with the oil consumption and ablowby gas from the abutting end portions.

[0016] (2) Since the piston ring is twisted in the use state when theundercut is large, it is necessary to form an inner cut in an innerperipheral side to establish a balance.

[0017] The present invention has been made in view of such drawbacks andit is an object of the present invention to provide a steel piston ringwhich exhibits superior oil scraping performance and good productivity.

DISCLOSURE OF THE INVENTION

[0018] To solve the above-mentioned problems, the present inventionprovides a steel piston ring which is characterized in that a groovehaving either an approximately quadrangular cross section with a lengthof each side of 0.05 to 0.4 mm or an approximately quarter-circularcross section with a radius of 0.05 to 0.4 mm is formed at a cornerportion defined by an outer peripheral surface and a lower-side surfaceof a piston ring body and the groove is continuously extended toabutting end faces.

[0019] It is preferable that the outer peripheral sliding surface has abarrel shape or a taper shape.

[0020] Further, in a combination of piston rings having a plurality ofpressure rings, the present invention preferably adopts a combination ofpiston rings in which at least two piston rings satisfy theabove-mentioned constituent features.

[0021] The present invention provides a combination of steel-made pistonrings which is characterized in that a first (top) ring includes agroove having an approximately quadrangular cross section with a lengthof each side of 0.05 to 0.4 mm at a corner portion defined between abarrel-face or taper-face outer peripheral surface and a lower sidesurface of a piston ring body, and a second ring includes a groovehaving an approximately quadrangular cross section with a length of eachside of 0.05 to 0.4 mm at a corner portion defined between a taper-faceouter peripheral surface and a lower side surface of a piston ring body,wherein the groove is continuously extended to abutting end faces.

[0022] Further, the present invention provides a combination ofsteel-made piston rings which is characterized in that a first (top)ring includes a groove having an approximately quarter-circular crosssection with a radius of 0.05 to 0.4 mm at a corner portion definedbetween a barrel-face or taper-face outer peripheral surface and a lowerside surface of a piston ring body, and a second ring includes a groovehaving an approximately quarter-circular cross section with a radius of0.05 to 0.4 mm at a corner portion defined between a taper-face outerperipheral surface and a lower side surface of a piston ring body,wherein the groove is continuously extended to abutting end faces.

[0023] It is needless to say that, from a technical aspect, it ispossible to provide versatile combinations of piston rings includingpressure rings having an straight face which satisfy the constitutionalfeatures of the present invention in at least two or all pressure ringsamong a plurality of pressure rings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a partial cross-sectional view showing a basic shape ofa barrel-face piston ring.

[0025]FIG. 2 is a partial cross-sectional view of the example shown inFIG. 1 in which a groove is formed at a corner portion.

[0026]FIG. 3 is a partial cross-sectional view showing a basic shape ofa taper-face piston ring.

[0027]FIG. 4 is a partial cross-sectional view of the example shown inFIG. 3 in which a groove is formed at a corner portion.

[0028]FIG. 5 is a partial enlarged cross-sectional view showing adetailed structure of the groove shown in FIG. 2.

[0029]FIG. 6 is a partial enlarged cross-sectional view showing anotherexample of the detailed structure of the groove shown in FIG. 2.

[0030]FIG. 7 is a partial enlarged cross-sectional view showing adetailed structure of the groove shown in FIG. 4.

[0031]FIG. 8 is a partial enlarged cross-sectional view showing anotherexample of the detailed structure of the groove shown in FIG. 4.

[0032]FIG. 9 shows an oil consumption amount (tests 1 to 4) under a fullload condition of 6000 rpm and a given mode condition (PATTERN: vehicleaverage speed of 100 km/hr) in a gasoline engine.

[0033]FIG. 10 shows an oil consumption (tests 5 and 6) using first andsecond pressure rings having a taper shape under the same condition asthose shown in FIG. 9.

[0034]FIG. 11 shows an oil consumption amount (tests 7 to 10) underconditions of a full load of 3600 rpm and a ½ load of 2000 rpm.

BEST MODE FOR CARRYING OUT THE INVENTION

[0035] The present invention adopts a following technique to suppress amachining amount of a lower side surface 3 and an outer peripheralsurface 4 of a piston ring 1 having a barrel-face or a taper-face outerperipheral sliding surface, that is, to suppress a machining amount forobtaining a corner portion 2 having a sharp edge to a small amount. Thatis, in the present invention, a groove 5 (see FIG. 2, FIG. 4, FIG. 5 andFIG. 7) having an approximately quadrangular cross section with a lengthof each side of 0.05 to 0.4 mm or a groove 6 (see FIG. 6, FIG. 8) havingan approximately quarter-circular cross section with a radius of 0.05 to0.4 mm is formed in the corner portion 2 defined by an outer peripheralsliding surface 4 and the lower-side surface 3 of the piston ring 1 byuse of lathe turning or grinding process at least by removing only aportion corresponding to an round portion remaining at the cornerportion 2. Here, the groove 5 is continuously extended to abutting endfaces without interruption.

[0036] Due to such a constitution, following advantageous effects can beobtained.

[0037] (1) At a lower end portion of the outer peripheral slidingsurface, a new corner portion which is constituted of the machinedportion and the outer peripheral surface forms a sharp edge 9 (see FIG.5 to FIG. 8) so that the reliable lubricant scraping performance isobtained.

[0038] (2) Since the machining amount of the side surface and the outerperipheral surface 4 of the piston ring can be reduced, the generationof burrs can be suppressed.

[0039] In addition to the above, it is also possible to obtain anadvantageous effect that the piston ring is not twisted even when such agroove is formed. Still further, it is possible to obtain anadvantageous effect that with respect to the groove of such a size, itis unnecessary to provide any means or measures to cope with the oilconsumption and a blowby gas in the vicinity of abutting end faces.

[0040] The reason why the length of one side of the groove 5 to bemachined having an approximately quadrangular cross section is set toequal to or more than 0.05 mm and equal to or less than 0.4 mm is that awire material for piston ring can be drawn and/or rolled only when around corner chamfering size of the wire material for piston ring fallsin a range of 0.05 mm to 0.4 mm. When the length of one side is lessthan 0.05 mm, the round corner of the wire material cannot besufficiently machined and hence, the sharp edge 9 cannot be formedwhereby it is difficult to obtain the sufficient lubricant scrapingperformance.

[0041] Further, an R size, radius of the corner portion 2 which remainsafter machining both side surfaces and the outer peripheral surface,even at a maximum value, does not exceed an corner radius R of theoriginal wire material and hence, the length of one side may be set toequal to or less than 0.4 mm. When the length of one side becomesgreater than 0.4 mm, the generation of the burrs is occurred and hence,the productivity is decreased to the contrary.

[0042] Here, the groove portion, that is, the machined-away portionaccording to the present invention substantially increases a volume of a2nd (second) land or a 3rd (third) land clearance between the cylinderbore and the piston although an increased amount is small. Accordingly,the oil pressure of the 2nd land or 3rd land clearance is reduced sothat the elevation of the lubricant to a top land or the second land canbe suppressed. That is, the groove portion plays a role of reducing theoil consumption.

[0043] In this context, the piston ring according to the presentinvention exhibits superior oil scraping effect (oil consumptionreduction effect) to a sharp edged product made of simple cast iron.

[0044] Further, it is preferable that a barrel-face surface or ataper-face surface is formed on the outer peripheral surface of thepiston ring. In this case, the groove is continuously formed withoutinterruption until the groove is opened at the abutting end faces.

[0045] Here, a linear portion 4′ formed on the outer peripheral surfaceis provided in a process of the outer periphery surface lapping. This isrecognized in careful observation.

[0046] The present invention is explained hereinafter based onembodiments.

[0047] [Embodiments]

[0048] As a first pressure ring of a gasoline engine, a steelbarrel-face ring (see FIG. 5 and FIG. 6) and a taper-face ring (see FIG.7 and FIG. 8) having a nominal outer diameter of Φ75×a width of 1.2 mm×athickness of 2.3 mm are produced.

[0049] As a second pressure ring of the gasoline engine, a steeltaper-face ring (see FIG. 7 and FIG. 8) having a nominal outer diameterof Φ75×a width of 1.5 mm×a thickness of 2.6 mm is produced.

[0050] As a first pressure ring of a diesel engine, a steel barrel-facering (see FIG. 5 and FIG. 6) having a nominal outer diameter of Φ99.2×awidth of 2.5 mm×a thickness of 3.9 mm is produced.

[0051] As a second pressure ring of the diesel engine, a steeltaper-face ring (see FIG. 7 and FIG. 8) having a nominal outer diameterof Φ99.2×a width of 2.0 mm×a thickness of 4.1 mm is produced.

[0052] These steel piston rings are served for engine tests to confirmthe advantageous effects of the present invention (see FIG. 9 to FIG.11).

[0053] A wire material used for production of piston rings is a steelcontaining 0.59 to 0.66% by weight of C, 0.15 to 0.35% by weight of Si,0.3 to 0.6% by weight of Mn, and equal to or less than 0.03% by weightof P and 0.03% by weight of S. For the first pressure ring of thegasoline engine, a rectangular wire material having a width ofapproximately 1.25 mm×a thickness of approximately 2.4 mm is used. Forthe second pressure ring of the gasoline engine, a rectangular wirematerial having a width of approximately 1.55 mm×a thickness ofapproximately 2.7 mm is used. For the first pressure ring of the dieselengine, a rectangular wire material having a width of approximately 2.55mm×a thickness of approximately 4.0 mm is used. For the second pressurering of the diesel engine, a rectangular wire material having a width ofapproximately 2.05 mm×a thickness of approximately 4.2 mm is used. Allwire materials have a corner R of approximately 0.30 mm.

[0054] Conventional products and the present invention products areproduced through following steps.

[0055] (Conventional Products)

[0056] barrel-face ring: coiling→heat treatment→side surfacegrinding→barrel face grinding→abutting-end-face gap machining→outerperiphery surface lapping

[0057] taper-face ring: coiling→heat treatment→side surfacegrinding→taper face grinding→abutting-end-face gap machining→outerperiphery surface lapping

[0058] (Present Invention Product) barrel-face ring: coiling→heattreatment→side surface grinding→barrel face grindinglower-surface-corner-portion machining→abutting-end-face gapmachining→outer periphery surface lapping taper-face ring: coiling→heattreatment→side surface grinding→ taper facegrinding→lower-surface-corner-portion machining→abutting-end-face gapmachining→outer periphery surface lapping

[0059] After completion of side surface grinding, the corner portion hasR of approximately 0.26 to 0.28 mm.

[0060] With respect to the conventional product, although the outerperiphery surface lapping is performed after the barrel face grinding orthe taper face grinding, the corner portion of the lower side surface ishardly machined and hence, the corner portion has R of approximately0.26 to 0.28 mm.

[0061] With respect to the present invention product, the lower surfacecorner portion 2 is machined using a coated carbide cutting tool suchthat a groove having an approximately quadrangular cross section with alength of each side of approximately 0.3 mm is formed in the piston ring(see FIG. 5 and FIG. 7). Further, besides such a piston ring, bychanging cutting tool, a piston ring having a groove wherein a crosssection of a machined portion is a quarter circular cross section havinga radius of approximately 0.3 mm is produced (see FIG. 6 and FIG. 8).

[0062] The above-mentioned piston rings are used as the first (top) ringand the second ring of a 2L gasoline engine and a 3.3L diesel engine.Oil rings for normal use are used in these tests. Under such conditions,advantageous effects of the present invention with respect to the oilconsumption are confirmed (comparing with the effects of tests 1, 5, 7which are directed to conventional ring combinations.) Here, theconventional piston rings are made of the same material and having thesame size as the present invention rings. (However, the conventionalrings are not provided with grooves 5, 6.)

[0063] A combination in which the barrel-face ring and taper-face ringare used as the first pressure ring and the second pressure ringrespectively in the gasoline engine is subjected to an oil consumptiontest under the full load of 6000 rpm or a given mode condition(PATTERN). A result of the test is shown in FIG. 9. A combination of theconventional rings is shown as a test 1.

[0064] It is confirmed that using the combination of the barrel-facepresent invention product as the first pressure ring and the taper-faceconventional product as the second pressure ring, the oil consumptionreduction ratio (to that of test 1) of 30% is obtained under the fullload of 6000 rpm, while the oil consumption reduction ratio (to that oftest 1) of 21% is obtained under the given mode condition (test 2).

[0065] Subsequently, it is confirmed that using the combination of thebarrel-face conventional product as the first pressure ring and thetaper-face present invention product as the second pressure ring, theoil consumption reduction ratio (to that of test 1) of 36% is obtainedunder the full load condition of 6000 rpm, while the oil consumptionreduction ratio (to that of test 1) of 30% is obtained under the givenmode condition (test 3).

[0066] Further, it is confirmed that using the combination of thebarrel-face present invention product as the first pressure ring and thetaper-face present invention product as the second pressure ring, theoil consumption reduction ratio (to that of test 1) of 45% is obtainedunder the full load condition of 6000 rpm, while the oil consumptionreduction ratio (to that of test 1) of 44% is obtained under the givenmode condition (test 4).

[0067] A combination in which the taper-face rings are respectively usedas the first and the second pressure rings of the same gasoline engineis subjected to an oil consumption test under the full load condition of6000 rpm or a given mode condition (PATTERN) . A result of the test isshown in FIG. 10. A combination of the conventional rings is shown as atest 5.

[0068] It is confirmed that using the combination of the taper-facepresent invention products as the first and the second pressure rings,the oil consumption reduction ratio (to that of test 5) of 43% isobtained under the full load condition of 6000 rpm, while the oilconsumption reduction ratio (to that of test 5) of 44% is obtained underthe given mode condition (test 6).

[0069] A combination in which the barrel-face ring is used as the firstpressure ring of the diesel engine and the taper-face ring is used asthe second pressure ring of the diesel engine is subjected to an oilconsumption test under the full load condition of 3600 rpm or a ½ loadcondition of 2000 rpm. A result of the test is shown in FIG. 11. Acombination of the conventional rings is shown as a test 7.

[0070] It is confirmed that using the combination of the barrel-facepresent invention product as the first pressure ring and the taper-faceconventional product as the second pressure ring, the oil consumptionreduction ratio (to that of test 7) of 22% is obtained under the fullload condition of 3600 rpm, while the oil consumption reduction ratio(to that of test 7) of 39% is obtained under the ½ load condition of2000 rpm (test 8).

[0071] Subsequently, it is confirmed that using the combination of thebarrel-face conventional product as the first pressure ring and thetaper-face present invention product as the second pressure ring, theoil consumption reduction ratio (to that of test 7) of 26% is obtainedunder the full load condition of 3600 rpm, while the oil consumptionreduction ratio (to that of test 7) of approximately 49% is obtainedunder the ½ load condition of 2000 rpm (test 9).

[0072] Further, it is confirmed that using the combination of thebarrel-face present invention product as the first pressure ring and thetaper-face conventional product as the second pressure ring, the oilconsumption reduction ratio (to that of test 7) of 36% is obtained underthe full load condition of 3600 rpm, while the oil consumption reductionratio (to that of test 7) of 58% is obtained under the ½ load conditionof 2000 rpm (test 10).

[0073] As can be clearly understood from the foregoing tests, thecombination of the first pressure ring and the second pressure ring, forexample, the combination of the piston ring forming the groove having aquadrangular cross section in the barrel-face and the piston ringforming the groove having a quadrangular cross section in the taper-faceshape is effective to reduce the oil consumption. Further, it is alsoconfirmed from the test results carried out in the same manner thatother combinations of piston rings shown in FIG. 9 to FIG. 11 can obtainthe high oil consumption reduction ratio.

1. A steel piston ring for an internal combustion engine, characterizedin that a groove having either an approximately quadrangular crosssection with a length of each side of 0.05 to 0.4 mm or an approximatelyquarter-circular cross section with a radius of 0.05 to 0.4 mm is formedat a corner portion defined by an outer peripheral surface and alower-side surface of the piston ring body and the groove iscontinuously extended to abutting end faces.
 2. A steel piston ringaccording to claim 1, wherein the outer peripheral sliding surface isformed in either a barrel shape or a taper shape.
 3. A steel piston ringaccording to claim 1 or 2, wherein the piston ring is a pressure ring.4. A combination of piston rings having a plurality of pressure ringsfor an internal combustion engine, characterized in that at least twopiston rings thereof satisfy the constituent features of claim 1 orclaim 2.